Activated alumina (aluminum oxide) and activated silica (silicon oxide) that have large surface area per unit volume are commonly used catalyst support materials. In conventional catalyst preparation methods, these catalyst support materials are mixed with precursors of the noble metal components by using three general methods, i.e. co-precipitation, deposition or impregnation. The mixtures are then subjected to a drying process to evaporate the solvent. The dry mass obtained is then subjected to a calcination process to convert the salt or hydroxide form of the active component into a metal or metal oxide form (in the case of aluminum oxide) by slowly heating the sample to the decomposition temperature.
A major challenge in preparing noble metal catalysts is in minimizing the amount of relatively expensive noble metals. The noble metal particles must be distributed (or dispersed) on support particles uniformly so that nearly all of the surface is exposed to the materials to be treated.
In the preparation of nanoparticle noble metal catalysts, a key element in the preparation process is to uniformly disperse the nanoparticles on the support substrates or support particles. Liquid phase preparation method for nanoparticles, when compared to the vapor phase preparation method, has numerous advantages such as simplified fabrication equipment, easily controllable noble metal particle structure and particle size. As a result, the liquid phase preparation method is more suitable in the mass fabrication of nanoparticles on support substrates.
In the conventional method for fabricating nanoparticles wherein support metal oxides are mixed with precursors of the noble metal components, it is difficult to obtain nanoparticles smaller than 10 nanometer due to the calcination process required to convert metal hydroxide to metal oxide. However, it is known that when the size of metallic nanoparticle becomes smaller than 10 nm, its catalytic efficiency improves dramatically. This is because of the quantum size effect, which is defined as the electronic properties of a nanoparticle start to change as its diameter approaches the exciton Bohr diameter.
It is therefore an object of the present invention to provide a fabrication method for nanoparticles that does not have the drawbacks or shortcomings of the conventional preparation methods for nanoparticles.
It is another object of the present invention to provide a method for fabricating nanoparticles that are smaller than 10 nanometers in size to be dispersed uniformly over metal oxide or inorganic oxide support particles.
It is a further object of the present invention to provide a method for fabricating nanoparticles of a noble metal catalyst by first converting a noble metal precursor to its metal phase and then supporting noble metal particles on a metal hydroxide prior to conducting a calcination process for converting the metal hydroxide into metal oxide.
It is another further object of the present invention to provide a method for fabricating nanoparticles of a noble metal catalyst that produces smaller sized noble metal particles and improved surface adhesion between the metal particles and the catalyst carrier.